The invention relates to an electronic trip device comprising a processing unit producing a tripping signal when currents of a power system to be protected exceed preset thresholds for preset times, and a trip storage device connected to the processing unit.
In state-of-the-art electronic trip devices, the tripping functions supply a circuit breaker opening order when at least one current flowing through a pole of the circuit breaker exceeds a preset current threshold for a time greater than that corresponding to the tripping curve. Among these functions, we can name the long delay function, or thermal function, and the short delay function, or magnetic function.
The long delay function has a threshold which corresponds to the circuit breaker setting current and a tripping time delay generally reversely proportional to the square of the fault current. In normal operation, with the circuit breaker closed, the long delay function simulates heating and cooling of a power system to be protected by making a quantity representative of the thermal state vary. After opening of the circuit breaker, the long delay function no longer operates, in the case of a system-powered trip device, and the thermal behaviour simulation is no longer performed.
Closing of the circuit breaker initializes the quantity representative of the thermal state at a minimum value and does not take account of the heat rise before opening of the circuit breaker and of the cooling during opening. This situation may be damaging for the power system if the opening was caused by a long delay fault trip and the circuit breaker reclosed on an overload, without a cooling delay.
To overcome these drawbacks, some state-of-the-art electronic trip devices comprise devices for storing the fault type indicating the thermal state before tripping, and devices for measuring the time elapsed between tripping and reclosing of the circuit breaker.
State-of-the-art storage and time measuring devices generally comprise a power supply circuit ensuring their operation during the open state of the circuit breaker. In state-of-the-art manner, the power supply circuits comprise capacitors, accumulator cells or batteries to conserve the electrical operating energy.
The electronic components to achieve these devices are costly and bulky, and become incompatible with highly integrated electronic trip devices able to be fitted in circuit breakers of small dimensions. Moreover, the reliability of the components, notably of the power supply circuits, deteriorates rapidly with time and with the ambient temperature of the circuit breakers.